Today's machines are relied upon to operate with minimal attention. Many industrial and commercial facilities operate hundreds or even thousands of machines concurrently, many of which are integrated in a large interdependent process or system. However, although maintenance procedures are becoming increasingly efficient, at any time at least a small percentage of the machines are prone to failure.
For example, machines having moving parts, such as bearings, are subject to constant friction that result in wear. Unfortunately, however, most wear sites are concealed in the machine's assembled state. In particular, bearing damage due to wear from inadequate lubrication, shock or lubricant contamination may not be immediately apparent absent gross damage and/or failure. Thus, it is difficult to monitor wear rates and to prevent excessive wear on internal components of a machine.
Vibration analysis is an established nonintrusive technique for measuring the health of mechanical components in rotating machines. Every rotating machine exhibits a characteristic vibration signature which varies with the design, manufacture, application and wear of each component. Vibration may be generated by machine bearings including, for example, the bearing races, balls and ball races, misalignment of gears, motors, or shafts, and imbalance of rotors, gears, pistons and fans. Analysis of a machine's vibration signature is valuable for reducing unscheduled down time, reducing turnaround time, minimizing periodic disassembly of a machine for inspection and greatly reducing the probability of catastrophic and unexpected machine failure.
A machine's vibration signature is composed of the sum of the vibration signals produced by and/or transmitted through each component of the machine. The vibration signals produced by a component include forcing frequencies that vary with the rotational speed of the machine. For example, the forcing frequencies for a bearing include those of the inner race, the outer race and the ball track, and can be calculated as a function of the rotational velocity, the ball diameter, the pitch diameter, the contact angle and the number of balls. The forcing frequencies are sometimes referred to as the critical frequencies. The health of a particular component can be analyzed by considering the shape and magnitude of the vibration signals at the critical frequency or at harmonics of the critical frequency.
Dynamoelectric machines such as motors generally have a shaft rotatably connected to the rest of the machine via at least two bearings, one toward either end of the machine. Prior to failure such bearings usually experience a degradation in performance which is accompanied by an increase in vibrations.
Since the bearings may be identical, it is generally difficult to determine from analysis of a vibration signal which of the bearings needs to be replaced when increased vibrations are detected. Thus the machine must be disassembled and more than one bearing examined in order to determine which bearing needs replacement. This disassembly and examination increases the time and expense involved in maintaining such machines.
It will be appreciated that it would be desirable to be able to determine, prior to disassembly, which of the bearings of a rotating machine is the source of irregular vibrations.